1. Technical Field
This invention relates to vibrating portable electronic devices, method of vibrating portable electronic devices and method of messaging by vibrating a portable electronic devices.
2. Discussion of Related Art
Vibration is an elegant way to unobtrusively inform a user of a portable device of an event, for example to inform a user of a mobile communications device of an incoming call or message. This vibration is easy to notice even in noisy conditions. Furthermore, a loud alarm signal is not necessary, especially since vibration can be used in combination with an audible alarm signal. In a typical vibrating mobile telephone, the reception of a short message or a telephone call can be indicated by vibration. The vibration is generated by using a miniature-sized electrical motor rotating an unbalanced weight.
The vibration of a portable device has two major parameters: frequency and amplitude of vibration. It is preferred for these parameters to be in a range within which the vibration is easy to detect, especially if the device is carried in a case or in a shoulder bag. Yet the vibration should not be unpleasantly vigorous.
Typically, in mobile telephones, the vibration is used in short pulses of approximately one or more seconds. For each pulse, the unbalanced weight is started and it accelerates to a nominal angular speed and causes the mobile telephone to vibrate. Initially, the unbalanced weight is stationary and it starts to accelerate when an operating voltage is applied to the electrical motor. Therefore, the start is “soft”, that is the frequency of the vibration increases and reaches a nominal value corresponding to the nominal angular speed.
In informing the user of an event by an audio alarm signal, the type of event may be indicated by using a different sound, melody, or even voice message. If vibration is used to inform the user of an event, it is difficult to distinguish between different vibrations. To distinguish one type of indication from another, one could alter the duration, the frequency or amplitude of vibration pulses. In each case, however, the change would need to be noticeable. Furthermore, changes in vibration frequency in a vibration pulse could easily feel unpleasant. On the other hand, the amplitude is difficult to vary without changing the frequency. The length of vibration pulses could be altered, but then it is difficult to detect accurately the start of a vibration pulse since the frequency of vibration gradually increases as the motor accelerates until the motor reaches a target angular speed.
Considering the underlying physics, the centrifugal force F causing vibration, for a mass element rotating about a rotational axis is:F=mω2rwherein m is the mass of the mass element, ω is the angular speed of rotation (radians per second) and r is the offset of the mass element from the rotational axis. As the equation is linear with respect to m and r, the equation can be generalized so that it refers to an infinitely small mass center of a real mass element that is not infinitely small. In this case, the mass m is the mass of the entire mass element and the radius r refers to the offset of the mass center from the rotational axis. The centrifugal force F causes thus a vibration force. Hence, the vibration force giving an amplitude of vibration (which then depends on the mass of the entire device to be vibrated) is linearly proportional to the mass and offset of the mass center, but proportional to the square of the angular speed (ω2) and, correspondingly, proportional to the square of the frequency of rotation. This explains why the amplitude of vibration is difficult to control by the rotational frequency, when inconvenient vibration frequencies are to be avoided.
In systems in which vibration is caused by rotating a rotatable mass about a rotational axis, one way to change the amplitude of vibration is by changing the rotatable mass to another, heavier or lighter, mass or to offset the rotatable mass further away from, or closer to, the rotational axis. Such operation is inconvenient for an average user of a staple portable device such as a mobile communications device.